GB2053757A - Lost wax patterns - Google Patents

Abstract

Means for reducing distortion in a wax pattern 10, e.g. for a gas turbine engine nozzle guide comprises moulding at least one wax supporting member 14 into the wax pattern which serves to support that portion 12,13 of the wax pattern liable to distortion. <IMAGE>

Description

SPECIFICATION Lost wax patterns This invention relates to wax patterns suitable for use in making castings by the lost wax process and more particularly to such patterns having an ability to resist distortion.

It is well known that it is possible by use of the lost wax process to manufacture castings to extremely close tolerances. This makes the process particularly suitable for producing large quantities of highly accurate castings for use in the manufacture of gas turbine engine components.

The process does however have disadvantages. In particular after the wax pattern has been injected it is removed from the die whilst it is still quite soft. This must be done to prevent the patterns cracking due to contraction of the wax and also to maintain an acceptable production rate. Certain wax patterns due to their relatively thin cross-section or complexity which are known to be susceptible to distortion are therefore placed in "setters" after injection where they are allowed to harden off. The setter is usually manufactured from plaster of paris and consists of a mould which conforms to the desired size and shape of the completed wax pattern.

Any distortion which occurs on the wax pattern after injection will therefore be eliminated by the setter. However it is usual to remove the wax pattern from the setter after approximately one hour so that the setter can be used to accommodate a further wax pattern. However the wax pattern after removal from the setter has a residual memory which remains for about one week, so therefore it is highly likely that the wax pattern will therefore distort again.

One way to overcome this problem is to allow the wax patterns to remain in the setters for a week.

However for mass-produced items this would necessitate an extremely large number of setters which would obviously prove expensive and would also create a considerable storage problem.

An object of the present invention is to provide wax patterns in which the aforementioned problems are substantially eliminated.

According to the present invention means for reducing distortion in parts of wax patterns comprises forming at least one wax supporting member upon the pattern during the wax injection stage, the at least one supporting member serving to restrain the parts of the wax pattern liable to distortion.

Preferably the at least one support can remain as a part of the pattern until after the casting has been made whereupon the excess metal formd by the at least one wax support member may be machined from the finished casting.

Alternatively the supports may be cut from the wax pattern prior to being used to form the ceramic mould for the casting.

For better understanding of the invention an embodiment thereof will be more particularly described by way of example only and with reference to the accompanying drawing in which there is shown a pictorial view of a wax pattern made in accordance with the present invention.

The drawing shows generally at 10 a pictorial view of a wax pattern suitable for manufacturing a gas turbine engine nozzle guide vane which comprises an aerofoil vane portion 11, and radially inner and outer platforms shown at 12 and 13 respectively. As can be seen from the drawing the platform portions 12 and 13 are relatively thin and weak and are obviously liable to distortion at their extremities. The platforms are therefore provided with supporting members 14, 15, 16 and 17 which are cast into the wax pattern during the wax injection stage.

In this example the supporting members 14, 15,16 and 17 are formed such as to simply extend between the two platforms 12 and 13. However in the case of the other shaped castings a supporting member or members may be located in any convenient location.

In certain circumstances it may be convenient to remove the supporting members just prior to the wax pattern being used for manufacturing the mould. In this way the wax pattern is supported for the maximum possible time without the necessity of using "setters". There is therefore little likelyhood of any distortion occurring in the brief time it takes to manufacture the mould from the wax pattern.

Alternatively the supporting members may be left as a portion of the wax pattern and therefore form a part of the resulting mould. It will therefore then be necessary to remove the supporting members from the completed casting by machining.

It will be appreciated that whilst the invention has been described as supporting members used for the manufacture of the nozzle guide vane for a gas turbine engine the scope of the invention is not restricted to such an application. The invention could equally well be utilised in the manufacture of any shape of casting produced by the lost wax process and in which the wax pattern is liable to distortion.

1. Means for reducing distortion in a part or parts of a wax pattern comprising forming at least one wax supporting member upon the pattern during the wax injection stage, the at least one supporting member serving to restrain the parts of the wax pattern liable to distortion.

2. Means for reducing distortion within a wax pattern as claimed in claim 1 in which the at least one support can remain as a part of the pattern until after the casting has been made, whereupon the excess metal formed by the at least one wax support member may be removed from the finished casting.

3. Means for reducing distortion within a wax pattern as claimed in claim 1 in which the at least one support may be cut from the wax pattern prior to it being used to form the ceramic mould for the casting.

4. Means for reducing distortion in a wax pattern as claimed in any preceding claim, substantially as hereinbefore described by way of example only and with reference to the accompanying drawings.

Printed for Her Majesty's Stationery Office by Croydon Printing Company Limited, Croydon Surrey, 19ûû.

Published bythe Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Lost wax patterns This invention relates to wax patterns suitable for use in making castings by the lost wax process and more particularly to such patterns having an ability to resist distortion. It is well known that it is possible by use of the lost wax process to manufacture castings to extremely close tolerances. This makes the process particularly suitable for producing large quantities of highly accurate castings for use in the manufacture of gas turbine engine components. The process does however have disadvantages. In particular after the wax pattern has been injected it is removed from the die whilst it is still quite soft. This must be done to prevent the patterns cracking due to contraction of the wax and also to maintain an acceptable production rate. Certain wax patterns due to their relatively thin cross-section or complexity which are known to be susceptible to distortion are therefore placed in "setters" after injection where they are allowed to harden off. The setter is usually manufactured from plaster of paris and consists of a mould which conforms to the desired size and shape of the completed wax pattern. Any distortion which occurs on the wax pattern after injection will therefore be eliminated by the setter. However it is usual to remove the wax pattern from the setter after approximately one hour so that the setter can be used to accommodate a further wax pattern. However the wax pattern after removal from the setter has a residual memory which remains for about one week, so therefore it is highly likely that the wax pattern will therefore distort again. One way to overcome this problem is to allow the wax patterns to remain in the setters for a week. However for mass-produced items this would necessitate an extremely large number of setters which would obviously prove expensive and would also create a considerable storage problem. An object of the present invention is to provide wax patterns in which the aforementioned problems are substantially eliminated. According to the present invention means for reducing distortion in parts of wax patterns comprises forming at least one wax supporting member upon the pattern during the wax injection stage, the at least one supporting member serving to restrain the parts of the wax pattern liable to distortion. Preferably the at least one support can remain as a part of the pattern until after the casting has been made whereupon the excess metal formd by the at least one wax support member may be machined from the finished casting. Alternatively the supports may be cut from the wax pattern prior to being used to form the ceramic mould for the casting. For better understanding of the invention an embodiment thereof will be more particularly described by way of example only and with reference to the accompanying drawing in which there is shown a pictorial view of a wax pattern made in accordance with the present invention. The drawing shows generally at 10 a pictorial view of a wax pattern suitable for manufacturing a gas turbine engine nozzle guide vane which comprises an aerofoil vane portion 11, and radially inner and outer platforms shown at 12 and 13 respectively. As can be seen from the drawing the platform portions 12 and 13 are relatively thin and weak and are obviously liable to distortion at their extremities. The platforms are therefore provided with supporting members 14, 15, 16 and 17 which are cast into the wax pattern during the wax injection stage. In this example the supporting members 14, 15,16 and 17 are formed such as to simply extend between the two platforms 12 and 13. However in the case of the other shaped castings a supporting member or members may be located in any convenient location. In certain circumstances it may be convenient to remove the supporting members just prior to the wax pattern being used for manufacturing the mould. In this way the wax pattern is supported for the maximum possible time without the necessity of using "setters". There is therefore little likelyhood of any distortion occurring in the brief time it takes to manufacture the mould from the wax pattern. Alternatively the supporting members may be left as a portion of the wax pattern and therefore form a part of the resulting mould. It will therefore then be necessary to remove the supporting members from the completed casting by machining. It will be appreciated that whilst the invention has been described as supporting members used for the manufacture of the nozzle guide vane for a gas turbine engine the scope of the invention is not restricted to such an application. The invention could equally well be utilised in the manufacture of any shape of casting produced by the lost wax process and in which the wax pattern is liable to distortion. CLAIMS

1. Means for reducing distortion in a part or parts of a wax pattern comprising forming at least one wax supporting member upon the pattern during the wax injection stage, the at least one supporting member serving to restrain the parts of the wax pattern liable to distortion.

2. Means for reducing distortion within a wax pattern as claimed in claim 1 in which the at least one support can remain as a part of the pattern until after the casting has been made, whereupon the excess metal formed by the at least one wax support member may be removed from the finished casting.

3. Means for reducing distortion within a wax pattern as claimed in claim 1 in which the at least one support may be cut from the wax pattern prior to it being used to form the ceramic mould for the casting.

4. Means for reducing distortion in a wax pattern as claimed in any preceding claim, substantially as hereinbefore described by way of example only and with reference to the accompanying drawings.

Printed for Her Majesty's Stationery Office by Croydon Printing Company Limited, Croydon Surrey, 19ûû.

Published bythe Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.